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Building a Javron Cub

At Bill's request I'm posting the following for informational purposes. At this stage of his weight and balance, it seems appropriate.

For your consideration and in recognition of your experience, I offer the following comments. In flight stability on a warm sunny day will be effected by thermal activity. A Cub on floats (also a 185 on large squared floats) only marginally passes the stability tests without a ventral fin. The overall stability is among other things a ratio of the tail volume to the wing span. My Cub with the extended squared off wings requires a lot of control manipulation in other than smooth air. I suspect that this small span increase, coupled with the large carb air filter, upsets the span/tail ratio. This becomes very tiring during a short bumpy flight. This is the reason for which I have installed the ventral fin from a Husky. Another contributor to the instability is a forward loaded CG. Take note that Piper moved the forward CG limit on floats aft 2.2". Perhaps this was their method of passing the stability tests without the ventral fin being installed?

In addition to the above, my experience with the Wipline amphibs on a Super Cub has shown that they also contribute to instability. Wip's method of choice to pass the tests was to place stops on the water rudders which increase the spring tension pulling the air rudder towards neutral. This only is an advantage at the one particular power setting and speed which coincides with that particular rudder position. This also assumes that the pilot does not use the rudder for coordinated turns. Another contributor to the instability is the squared shape of the floats which generates destabilizing forces in the bows. Neither of these two Wip (discrepancies) characteristics do anything to increase the tail volume which is necessary for the real improvement to stability. I acquired a field approval from the FAA to remove these water rudder stops on a stock PA-18 by installing a Ventral fin. This made a great improvement in stability and control.

The round top EDO 2000 and Montana amphib floats are much more stable on a stock Cub than the Wips.

I mention this since you are planning a nice long trip to Alaska. When flying a loaded amphib on a long trip in the summer thermals it quickly becomes tiring in an unstable flying machine. I installed a yaw damper in my 185 to reduce this instability. None of the above mentions that there will be a higher cruise speed with the CG loaded aft when coupled with a trimming stabilizer.
 
Bill,

You continue to be a major motivator here and with the help of your friends have built a quality and weight standard for us all!! When are you starting the next one...keep up the great work!?
 
From the pictures, the tail weighing point is clearly the tailwheel, not the tailpost.

John Scott
I was thinking of the factory "Airplane Weighing Diagram" showing the factory weighing points. Piper used the tailpost but it was 186" from the main wheels. The tailwheel. Piper shows the center of gravity of the tailwheel at 200" but looking at the Scott drawing it shows the CG of the tailwheel way forward of the wheel centerline. Probably splitting hairs but the ones I have plumb bobbed have been between 202.25 and 202.5" aft of the main wheels when level.
 
Steve

Your numbers pretty much agree with mine. Tail post at 186, but my tail wheel axle bolt I show at 203.69 but I have a Pawnee spring so that will probably alter things a bit. And I agree, we are splitting hairs. "Measure it with a micrometer, cut it with an ax". LOL

Bill
 
Steve

Your numbers pretty much agree with mine. Tail post at 186, but my tail wheel axle bolt I show at 203.69 but I have a Pawnee spring so that will probably alter things a bit. And I agree, we are splitting hairs. "Measure it with a micrometer, cut it with an ax". LOL

Bill

Don't forget to mark it with chalk.
 
Steve

Your numbers pretty much agree with mine. Tail post at 186, but my tail wheel axle bolt I show at 203.69 but I have a Pawnee spring so that will probably alter things a bit. And I agree, we are splitting hairs. "Measure it with a micrometer, cut it with an ax". LOL

Bill
Bill with your "Helium Cub" why a Pawnee Spring?
 
Pete is dead right regarding the float installations on these aircraft. Note that the CC 18-180 "Top Cub" requires a ventral fin on Wipline floats. That airplane needs a fin.

my experience with Cubs with extended/squared off wings on floats mirrors Petes observations.

I would definitely fabricate a fin for that airplane for float use. That will also put a couple pounds of USEFUL weight back aft as well, to counter the fwd weight of the floats.

MTV
 
Pete is dead right regarding the float installations on these aircraft. Note that the CC 18-180 "Top Cub" requires a ventral fin on Wipline floats. That airplane needs a fin.

my experience with Cubs with extended/squared off wings on floats mirrors Petes observations.

I would definitely fabricate a fin for that airplane for float use. That will also put a couple pounds of USEFUL weight back aft as well, to counter the fwd weight of the floats.

MTV

But they are so........ butt ugly

Glenn
 
This would be a good opportunity to make a removable ventral fin that combines a bunch of things. Stabilizing area moved as far aft as possible, a tail tie down attachment location, a ballast container and good looks. Use your imagination keeping in mind that one purpose is to add weight. Bill, remember that this is one place where you will want weight.

Be sure to make the alignment adjustable as the fin is sensitive to directional trim. It may take a few test flights to trim the ball in the center. Once set, it can be forgotten.
 
Hey Bill,

Just wanted to pop in and say what a great job you're doing! We've been following along silently :peeper and are excited to see the finished product. Keep up the good work! :up
 
Kirby - Because it fits the mission. I think it might be easy to read this thread and think I'm trying to build an "ultralight" cub. I'm trying to build a light weight hard core, off runway, go to Alaska on floats, heavy duty, gravel bar Cub. I have reinforced- heavy duty, extended landing gear, double puck brakes, 1.5" axles, reinforced tail, extra tubing in the tail, pawnee spring, X braced birdcage, square wings reinforced to handle a 2300 pound GW, X braced firewall, upper and lower baggage to haul a quartered out moose, 0-360 engine, essentially IFR instruments, provisions for a fuel pod, extra weight in seat belts, float fittings, etc etc. This is one reason I am so pleased with the weight. This is not a stripped down C-90 powered Cub.
I hope, (I will find out soon) that I have built a light Cub that is fun to fly locally but is also very capable of full up, hard core, adventure flying.

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Putting the mineral oil in. I am hoping for the initial engine run tomorrow.


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Still putting in the labels. As you can see we have not done the fuel calibration and labeling yet. Tomorrow. But the reason I took this photo was to remind builders that you HAVE to have this decal in easy view of all occupants. Don't forget, you also need the data plate on the tail.


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Hey Mike, as you can see I replaced the cowl brace rod adell clamps with the proper ones. Thanks for the tip.


Hope this helps


Bill
 
What... you didn't have the oil in for empty weight... add 10 more lbs to the nose and declare a new weigh in winner! :smile:
 
Are you supposed to weigh the plane while someone is sleeping in it? I would think that extra weight is not desired.:lol:

You have enough weight for the 454 cassull, and the shotgun if you wanted it Bill.

Pretty cub, bring it up and lets spray it with blood and slime!
 
Bill,

You have accomplished the all desired utility cub (and wecan sometimes forget your mission) at very low weight, something which can't be replicatedin the certified world, which for business reasons I'm stuck in. My mostlystock 150-SC (pre-tree encounter) was about 1085 on ABW’s (both ends) plusOregon Aero seats, mid-sized extended baggage, nothing X'd but with inertiareel belt and very simple panel. The CG was dead center and the tail waslighter than anyone around me but with that anything heavier than a properlycurved stock tail spring and it rode really hard back there transmitting a lotof shock load to the frame especially empty. Only when in travel mode with apassenger and a full baggage (below 1750 and in CG) did I ever have shimmyissues and then only if you put the tail down hard. With my rebuild we addedall of the stiffening we could think along with the increased GW we had to goto the heavy spring but at light loads on rough stuff I have concerns about transferringshock loads. I guess my concern is that and we'll be in the same boat but I'mnot headed to Alaska anytime soon and you had choices but the only realsolution is for a better system back there...

Kirby
 
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Kirby

Well said. Yes, I have some concerns about the transfer of shock and loads. Jason Gerard - unfortunately for us all, no longer active in the Cub community - was a strong proponent of the stock spring. As you say there are advantages and disadvantages for everything. Sounds like your old Cub was pretty light. It is tough to keep em light. Ya'll have a good time at OK18 and Lord willing I'll be there next year in my Cub.


Pete - Thank you for that knowledgable and informative post on float stability. Good stuff. I will definitely consider a fin.

George - Lord willing I'll see you in late August.

OregonAero - great to see you on the forum. You guys make a great product. I understand you are working with Javron so you new Javron builders can order your Oregon Aero seats directly from Javron. Pretty cool.

OK - got my daily Aircraft Spruce order in - back to work

Bill
 
Thanks Bill! :lol: That's right you guys can get our seats right from Javron now, but if you ever have any questions feel free to give us a call 800.888.6910. We're very excited to see this Cub in the air!
 
Bill, where'd you get that throttle ball with the PTT switch? Is that an EASY button on there? ;-)

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Soy - don't know but trying to get the info for you. A friend picked that up at the local electronics store and will go back and get the info. It is a standard Piper throttle ball.

Folks - no engine run yesterday. Should get it today. Really coming together now.


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Got the data plate etched. Tried to find a data plate with a little more info than name and aircraft. I know when I look at a plate I like to get some info so figured others might feel the same way. Also put the kit manufacturer's name in there.


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Made my own spark plug wire pass throughs. The ones from ACS are stupid expensive. 24.95 for a small piece of plastic. I grabbed a piece of UMHV laying around the hangar, 1/4" thick, drill the pass through holes first, then cut it in half. When drilling it for attachment to the baffling lock it down tight. It is slippery and wants to move around. I think it will work just fine.


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Back side so you can see the cut out to allow the spark plug cap to pass through


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Here is another thought. The little pointer on these fuel selector valves is just not very intuitive and some use the handle as the selected position indicator. I ground the pointer off and will put a little data plate warning that the handle is the selector. It also made the positions more logical. This would all be exactly the opposite if it had a pointer ie the right tank would be on the left side of the plate, off would be at the top etc. Also we used air back through the carb line to make sure we had the valve positions correct. Just be SURE to lower the air compressor to about 10 psi or less when you do this.

Hope this helps. Perhaps some engine run and other photos tonight.

Bill
 
I also use that UHMV stuff a lot for misc., a good source for it when you need it NOW is any supermarket. They call it a cutting board but we know better!
 
Your going to love those auto spark plus ...new set every oil change.
 
Your going to love those auto spark plus ...new set every oil change.
And why do you recommend a new set at every oil change? Just because they are inexpensive is not the answer that I am looking for. I've been through three oil changes on my auto plugs so far and they are still in excellent operating condition.
 
I'm using NGK BR8EIX per Fedex Lou's recommendation. When I replace them I'm going to try the BR7EIX which is up one heat range as an attempt to eliminate a very small amount of lead accumulation.
 
I put over 600 hrs, twice and coming up on a third time, on the NGK auto plugs in my homebuilt, before replacement. They still looked like new (pretty much), zero performance change with new ones anyway. I check the gap every 300 hrs, that's it.

FWIW: 100% mo- gas and a Rotax 912S, so not apples to apples of course.
 
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First time in the sun. It runs good, just a short run to check it out. No oil leaks, fuel leaks, no smoke or sparks. Yahoo......


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A few clean up items and we are ready for inspection on Tuesday.

More later

Bill
 
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